Vibrating reed vibrometer



R7., Sl ou Aug, 25, 1959 E. l.. GRINDLE ETAL 2,900,817

VIBRATING REED VIBROMETER Filed March 2, 1955 Y f-R l/ ,w A, E

United States Patent VIBRATIN G REED VIBROMETER Eugene L. Grindle, San Mateo, and James L. Savage,

I Iedwood City, Calif., assignors to Deterjet Corporation, San Mateo, Calif., a corporation of California Application March 2, 1955, Serial No. 491,586

4 Claims. (Cl. 73-70.2)

This invention relates to vibrometers and particularly to vibrometers of the vibrating reed type. The invention further relates to apparatus for detecting excessive propeller vibration and apparatus for phasing propellers of multi-engine aircraft.

Conventional vibrometers have been found to be expensive and not suited for particular applications such as used in aircraft. In general, itis an object of the present invention to provide ya new and improved vibrometer which is particularly suited for such use.

Another object `of the invention is to provide a vibrometer of the above character in which a solid metal-tometal contact is provided at the fulcrum point of the vibrating reed.

A further object of the invention is to provide a new and improved vibremeter of the above character in which the vibrating reed is continually urged in one direction at the fulcrum point.

A further object of the invention is to provide a new and improved vibrometer of the above character with a scale for measuring frequency of vibration and amplitude of vibration.

A further object of the invention is to provide a method for detecting excessive propeller vibration with a vibrometer.

' A further object of the invention is to provide a method for phasing propellers of multi-engine aircraft with a vibrometer.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawing.

Referring to the drawing:

' Figure 1 is an elevational view of a vibrometer incorporating the present invention;

Figure 2 is an enlarged cross-sectional view taken along Ithe line 2-2 of Figure 1;

Figure 3 is a partial enlarged elevational view of our vibrometer with the cover removed;

Figure 4 is a cross-sectional view taken along the line 4 4 of Figure 3; and

Figure 5 is a view similar to Figure 3 with misalignment of the reed exaggerated.

The embodiment of our device as shown in the drawing consists of an L-shaped mounting plate 11 which forms a support for mounting block 12 and graduated scale 13. Mounting block 12 may `be secured to plate 11 by any suitable means such as screws 14.

A fulcrum block 16 is attached to the upper portion of mounting block 12 as by screws 17 and is provided with an elongate slot 18 and a ll-shaped slot 19. One end of the depending arm 21 extends into slot 19 and is pivotally carried by a pin 22 that extends between the fulcrum block 16 and the mounting block 12. A roller 23 is rotatably mounted on the other end of depending arm 21. The lower end of the arm 21 and the roller 23 are continuously urged to the right as viewed in Figure 3 ICC by a spring 24 that is mounted on rod 22 for a purpose hereinafter described.

A thumb screw 27 is rotatably mounted in block 12 adjacent roller 23 and is provided with a rubber roller 28. A vibrating reed 31 is frictionally carried between rollers 23 and 28 and passes through elongate slot 18 in fulcrum block 16 and through a slot 32 in the lower portion of L-shaped mounting plate 11. Slots 18 and 32 are aligned vertically but thumb screw 27 and roller 28 have been oifset slightly to the left (exaggerated in Figure 5) as viewed in Figure 3. This causes bending of reed 31 about roller 28 and urges the reed against the lower left side of slot 18 and the upper right side of slot 18 as viewed in Figure 3. Bending of the reed causes the portion of the reed above block 16 to be swung to the right as viewed in Figure 3 from the vertical centerline formed by slots 18 and 32. This is overcome by shifting block 12 slightly counterclockwise as viewed in Figure 3 until the upper portion of reed 31 rests on the aforementioned vertical centerline. The force exerted by the roller 28 on the reed 31 is greater than the greatest force introduced into the reed by vibration of the reed and hence the reed will always remain in engagement with the upper right side of the slot 18.

Spring 24 and roller 23 increase the friction between roller 28 and reed 31 and assure positive slip-free adjustment of the reed length without damping vibrations.

A stop 33 has been placed on Ithe bottom portion of reed 31 and is slightly larger than slot 32 to thereby prevent drawing that end of the reed through the same. The other end of the reed has been provided with a tip 34 as a visual aid to facilitate the reading of scale 13 and which also serves as a stop to prevent that end of the reed from being drawn through slot 18.

The scale 13 has been calibrated vertically in cycles per minute and horizontally in amplitude of vibration in thousands of an inch double amplitude of vibration.

A cover 37 has been provided to protect the working parts. It may be of lany suitable transparent material such as plastic and is provided with a pair of grooves 38 which are adapted to engage the side edges of the L- shaped mounting plate 11. The lower portion of cover 37 is provided with a slot 39 to allow cover 37 to accommodate thumb screw 27. The lower portion 41 of the cover may be coated with suitable opaque material to conceal the mechanism for varying the length of the reed.

rPhe operation of the device may now be briey described as follows: Assuming that it is desired to obtain the vibration frequency and amplitude of vibration of a piece of equipment, particularly where rotating elements are being used such as in an airplane, the base of the vibrometer is placed on a rigid portion of the apparatus that is not shock mounted. The vibrator is then tuned by extending or retracting the free end of the reed by rotating thumb screw 27 until the reed vibrates at maximum amplitude. As is Well known to those skilled in the art, the natural frequency of Vibration of the reed is dependent on its length. Therefore, with a proper scale, the vibration frequency of the reed may be read directly upon the scale at the free tip of the reed. For example, as shown in Figure 1, if the reed is vibrating at maximum amplitude at the length shown, it is vibrating at the frequency of 600 cycles per minute.

After Ithe frequency of vibration has been ascertained, the amplitude of vibration may be determined from the vertical lines on the scale 13.

In measuring low frequencies, the reed occasionally may be excited by some frequency higher than that corresponding to the reed length. When this happens, a node will appear between the fulcrum and the free end of the reed. To obtain a true reading, the reed should be shortened until the end vibrates freely with a maximum amplitude free of nodes.

It is apparent that our device is suitable for obtaining the frequency of vibration and the amplitude of vibration of various types of equipment, particularly those utilizing rotating parts. The mechanism we have shown for advancing and retracting the reed adds to the accuracy of our device. As shown in Figure 3, the reed 31 passes through the slot 18 and is continuously urged against the upper right side of the slot as viewed in Figure 3 to provide a metal-to-metal contact for the fulcrum point of the reed. This mechanism serves to maintain the fulcrum point of the reed at one particular point regardless of the length of the reed extending above the fulcrum block 16. 1

We have also provided a scale in which the frequency'. of vibration and amplitude of vibration can be read atA the same time.

' Our device is particularly suitable for use in a method for detecting excessive propeller vibration. It is Well known that rotating components on an airplane will vibrate at frequencies in relation to their basic rotating frequencies. An engine turning at 2,000 r.p.m. may vibrate once per revolution, in which case the source of Vibration would probably be the crank shaft. It may vibrate once every two revolutions, in which case the the source of vibration would probably be a cylinder. A'propeller will usually vibrate once per propeller revolution or a por- We do not, however, eliminate the actual vibration of any particular propeller, but only the additiveeifects of alll l that the propellers have been synchronized by the autotion of the crank shaft speed directly dependent upon thel (A gear reduction ratio. However, it is common to find vibration frequencies at propeller speed times the num# ber of blades. This is caused by air slugs from each blade. The source of Vibration can be isolated by the method hereinafter set forth by identifying the vibra-k tion frequency with a rotating component of a directly related frequency.

The vibrometer is mounted on any suitable portion of the aircraft which is not shock mounted and preferably in the cockpit within easy reach of a crew member. The reed of the vibrometer is then tuned until it vibrates at the maximum amplitude. If the amplitude of vibration or deflection exceeds a predetermined value such as two thousandths of an inch double amplitude, corrective action may be required on the aircraft. The vibration frequency is then identified with the speed of some aircraft component frequency by multiplying the engine speed by one of the following:

(a) Crankshaft (b) Power impulses or cylinder reciprocations (c) Propeller-by the gear reduction relation between crankshaft and propeller, e.g. .375

As soon as the aircraft frequency component has been determined, the particular engine that is causing the trouble is isolated by advancing the r.p.m. on one engine at a time while holding the others at the vibrating frequency. When the increase in speed of a particular engine results in a noticeable reduced amplitude, that engine is at fault.

Our vibrometer has also been found to be particularly useful in a method that we have devised for phasing propellers of multi-engine aircraft for minimum vibration. It is well known that on multi-engine aircraft it is desirable to adjust all engines to the same speed after the cruising altitude has been assumed. When two or more propellers are operating at different speeds, vibration and throbbing noises may be set up. Various types of engine synchronizers have been developed by which the pilot, by operating the propeller controls, causes the propellers to rotate at exactly the same speed. However, it has been found that although the propellers are rotating in synchronism, considerable vibration may still be imparted to the plane.

By our method, it is possible to reduce the vibration level in the plane to an almost imperceptible matic synchronizing means hereinbefore described, the vibrometer is tuned to maximum amplitude at propeller frequency. The propeller synchronization selector switch is placed in manual. The number 2 propeller toggle switch is actuated in one direction and then the other direction to obtain a very slow rotation of number 2 propeller yin relation to the other propellers. The propeller toggle switch serves to vary the pitch of the propeller and hence the propeller speed at a particular power input. d Asthe number 2 propeller is rotated through 360 with respect to the other propellers, the vibrometer reed will begin to increase or decrease in amplitude. When the vibrometer reed'has reached a minimum amplitude, the synchronization selector switch is moved to the automatic or synchronized position. The number 2 propeller will now be locked in a new relationship with the other three propellers.

The same procedure is then followed with the other propellers until the propellers are phased to give minimum slidably mounted in said slots, and roller means friction,

ally engaging a portion of the reed between the slots, the roller means being so positioned that the point of contact between the reed and the roller means is out of alignment with said slots to cause said reed to be urged against one side of said slot to provide a fulcrum for said reed.

2. A vibrometer `as in claim 1 together with means for increasing the friction between said roller means and said reed, said means including a roller engaging the reed on the side opposite the side engaged by the roller means.

3. A vibrometer as in claim 2 together with means prov vided at each end of said reed to prevent the ends of said reed from passing through said slots.

4. A vibrometer as in claim 2 together with a scale con,-

.nected to said mounting block adjacent the free end of said reed, the scale being calibrated in frequency of vibration and amphtude of vibration to permit simultaneous readlng of frequency of vibration `and amplitude of vibration.

References Cited in the lile of this patent UNITED STATES PATENTS 2,306,391 Keinath Dec. 29, 1942 2,352,219 Olesen June 27, 1944 2,361,990 Brown Nov. 7, 1944 2,428,671 Kent et al. Oct. 7, 1947 2,496,295 Langer Feb. 7, 1950 2,511,694 Burkhardt .lune 13, 1950 2,514,153 Dickerson July 4, 1950 2,604,781 Bechtold et al Jul-y 29, 1952 2,711,647 Ongaro et al. -..June 28, 1955 

